Over the history of development of switched mode power supply systems (SMPS) or resonant converters, the rectifier circuit side of the converter has increased in complexity.
In traditional resonant SMPS, the rectifier circuit portion of the SMPS included conventional semiconductor diode rectification components, which rectified the voltage signal produced by the resonant tank portion of the converter. In order to control the output voltage of the converter, the frequency at which the resonant tank was switched was varied.
FIG. 1 shows a typical known circuit topology of a series resonant LLC converter with conventional diode rectifiers. A DC input voltage source 101 is provided to a resonant tank 103 (a series inductor (Lr) and capacitor (Cr)) through the operation of two switches (105A, 105B), which are controlled by a variable frequency oscillator 107. By adjusting the variable frequency, the impedance of a second inductor 109 (Lm) is changed to adjust the current that flows through the resonant tank 103. This in turn causes the output voltage (Vout) to be controlled. That is, the transfer function of the converter is frequency dependent. An isolation transformer 111 is used to isolate the input and output portions of the circuit. The conventional diode components (113A, 113B) rectify the current produced on the secondary side of the isolation transformer 111 and feed this current to an output capacitor 115, which is used to smooth the output voltage signal.
In order to make this form of converter more efficient, synchronous rectification was introduced. The conventional rectifier diode components were replaced with synchronous rectification (SR) MOSFET (Metal Oxide Semiconductor Field Effect Transistor) devices. The MOSFET devices were controlled to emulate the operation of the conventional semiconductor diodes by driving the gate nodes of the MOSFETS.
For example, a typical SR circuit is shown in FIG. 2, where two MOSFET devices (201A, 201B) have replaced the conventional diode rectifier components (113A, 113B) of FIG. 1. Apart from the conventional diode components, the circuit in FIG. 2 contains the same components as shown in FIG. 1 and the same reference numerals have been used. An example method of controlling such a SR circuit is to turn the first MOSFET device 201A ON using a gate drive signal 203A when it is desired that the device is to emulate the conventional diode in a forward conducting mode, i.e. the MOSFET device is switched ON so that the channel conducts in the reverse direction from source to drain. The first MOSFET device 201A is then switched OFF using the gate drive signal 203A to stop the channel from conducting in a forward direction from drain to source, thus causing the MOSFET device to act as a diode in reverse bias mode. The second device 201B may then be operated in a similar manner using gate drive signal 203B for the opposing portion of the signal fed from the isolating transformer.
However, the introduction of a switching control system for the synchronous rectification has led to more complex control mechanisms. Both the turn-on and turn-off times for the SR transistors need to be accurately controlled in order to achieve the maximum efficiency gains afforded by incorporating SR transistors while ensuring that the SR transistor switching does not adversely affect the output voltage control.
Further, although the inclusion of a second inductor (Lm) has provided additional control over the output voltage, this has led to increased production costs as well as resulting in more bulky systems due to the extra inductor component required.
Also, systems using the above described approaches are generally rated or designed for operation for a defined maximum voltage. The control of the system using the above described techniques allows the converter to operate at a lower voltage than the rated maximum voltage. However, operation of the converter at a voltage less than the maximum rated value results in the system not operating in the optimal range, thus losing efficiency.
An object of the present invention is to provide a resonant converter circuit with a simpler control system.
A further object of the present invention is to provide a resonant converter circuit with a reduced number of inductors.
A further object of the present invention is to provide a method of controlling a resonant converter in a simpler manner.
Each object is to be read disjunctively with the object of at least providing the public with a useful choice.
The present invention aims to overcome, or at least alleviate, some or all of the afore-mentioned problems.